In our UK application No. 8017432, published as GB-A-2053779, we have described a building board comprising a core of set cementitious material, such as gypsum, faced on at least one side by a fibrous sheet embedded in the face of the core, and a continuous film of set cementitious material having a higher density and lower porosity than the core extending over the outer face of the sheet. Where the sheet is composed of glass of other mineral fibres, such board has improved performance in fire. When mounted on conventional supports or studs and subjected to fire testing, however, it is found that the structure will eventually deform or collapse owing to deterioration of the supports, while the panels are still intact. Even in the case of steel studs, considerable distortion can occur after a time while the panels themselves remained relatively unharmed.